1. Field of the Invention
The present invention relates to a protective relay designed to protect a transformer which is an essential component device in a power system, and more particularly to an improved relay equipped with a second harmonic suppressor for preventing a malfunction that may be induced by a second harmonic generated at the time of inrush or the like.
2. Description of the Prior Art
FIG. 1 is a block diagram of a conventional relay of this type known heretofore, wherein there are shown a suppression input terminal 1; a differential input terminal 2; a suppression circuit 3; a differential circuit 4; a first comparator circuit 5; a ratio differential element consisting of the said suppression circuit 3, differential circuit 4 and first comparator circuit 5; a fundamental-wave extraction circuit 7; a second-harmonic extraction circuit 8; a second comparator circuit 9; a second-harmonic detection element consisting of the said fundamental-wave extraction circuit 7, second-harmonic extraction circuit 8 and second comparator circuit 9; an AND circuit 11 which serves to provide the output of the ratio differential element 6 only when the output of the second-harmonic detection element 10 is not received; and an output terminal 12.
There are also shown in FIG. 1 a differential input I1, an output a of the ratio differential element 6, a fundamental wave I2 in the differential input, a second harmonic I3 in the differential input, an output b of the second-harmonic detection element, and a final output c. The respective waveforms thereof are illustrated in FIGS. 2 and 3, wherein the amounts of AC components are represented by dotted lines while the amounts of rectified current components are represented by solid lines.
The operation of such a conventional relay will now be described with reference to FIGS. 2 and 3.
The waveform chart of FIG. 2 relates to an exemplary case where an inrush current of a transformer flows as a differential input. An inrush current is generated upon energization of the transformer without any load is therefore not a fault current, so that the protective relay associated therewith should not function in response to such an inrush current. Since a considerable amount of second harmonic component is normally contained in the inrush current, it is generally customary to employ a system of suppressing the output by detecting the content of the second harmonic component. The device of FIG. 1 also adopts such output suppression system. As illustrated in FIG. 2, the output a of the ratio differential element 6 is active when the differential input is above a predetermined value, while the output b of the second-harmonic detection element 10 is active when the ratio between the second harmonic I3 and the fundamental wave I2 is high. Therefore, the output c of the AND circuit 11 is not provided due to the fact that the output condition of the AND circuit 11 is not satisfied because of the presence of the output b, although the output c rises as represented by a dotted line in case the output b is absent.
The waveform chart of FIG. 3 relates to another exemplary case where a differential current is generated as a result of an internal fault, showing that a fundamental wave I2 is generated when a fundamental-frequency current flows as a differential current I1 at time t1. In general, the fundamental wave I2 is delayed in its rise due to a filter employed in the fundamental-wave extraction circuit 7 and therefore increases gradually as illustrated. Considering that the input is composed essentially of the fundamental wave I2 alone, the second harmonic output I3 is not to be generated. However, since the second-harmonic extraction circuit 8 also incorporates a filter whose characteristic is usually more acute than that of the fundamental-wave extraction circuit 7, the output I3 comes to present steep increase and gradual decrease eventually because of the transient phenomenon derived from the input variation. It follows that the output a of the ratio differential element 6 becomes active when the differential input is above a predetermined value, but the output c of the AND circuit 11 is delayed in its rise as the output b of the second-harmonic detection element 10 obtained in relation to such second harmonic output I3 is sent for a fixed period of time in the initial stage. This is based on the reason that the operation time is prolonged by the second-harmonic detection element 10.
An explanation will now be given with regard to an instance where the differential current is suddenly reduced at time t2 in the waveform chart of FIG. 3. When there occurs a sudden reduction in the differential current I1 at time t2 within the operating zone of the ratio differential element 6, the output is supposed to be sent continuously since the shift of a fault point is included within the operating zone. However, the second harmonic output I3 varies to present a mountain-shaped waveform while being affected by the transient phenomenon in the second-harmonic extraction circuit 8 derived from the input variation, and in case the ratio between the second harmonic output I3 and the fundamental wave I2 is greater than a predetermined value, the output b of the second harmonic detection element 10 comes to rise, whereby the output c of the AND circuit 11 is temporarily interrupted. The above-described shift of the differential current within the operating zone of the ratio differential element 6 occurs upon generation of a system fault during inspection of the protective relay with application of a checking current, or at the time of removing the checking current posterior thereto. (The checking current should be removed for interruption of the inspection during which the action of the protective relay fails to provide an effective output for energizing the coil of a breaker.) Thus, there has been a disadvantage heretofore that the occurrence of such current shift renders setting of the inspection sequence difficult.